The invention relates to a thrust piece unit for rack-and-pinion steering mechanisms.
In thrust pieces of a conventional form of construction, as described in DE 34 08 673 A1, the thrust piece presses against the rear side of the rack by means of the resilient force of a spring, in order to ensure play-free meshing between rack and pinion. In this case, the spring is supported against the inside of a screwable cover, with a limited gap (play) being maintained between cover and thrust piece. The play and the resilient force of the spring can be set by the amount by which the cover is screwed in, and, when the thrust piece is subjected to load by the rack, the impact is damped according to the presetting.
Over the period of useful life, because of high forces, such as arise, for example, in the case of electromechanical rack-and-pinion steering systems, increased wear occurs on the toothing and on other components matched with one another, with the result that the gap is enlarged. The increased possibility of axial deflection of the thrust piece afforded thereby, may lead to a disturbing noise behavior, in spite, for example, of a spring between thrust piece and setscrew.
In a thrust piece unit disclosed in DE 36 37 107 A1, in order to ensure the play-free meshing of the pinion in the rack, the thrust piece, in addition to being prestressed by a spring in the conventional way, is acted upon hydraulically. The thrust piece unit is integrated in a bore of the steering mechanism housing. Said unit consists of a cylinder part and of a piston part, the cylinder part having a partition which is equipped with an nonreturn valve arranged in a throughflow orifice and with a throttle bore. The piston part, acted upon with force by a spring, is guided on the rack-facing side of the cylinder part and encloses a pressure space. On the opposite side of the partition, an adjusting piston likewise acted upon by a spring encloses a storage space. Since the pressure space is filled completely with hydraulic fluid and is closed with respect to the storage space by means of the nonreturn valve, a hydraulic cushion against the rack being lifted off from the pinion is formed. Wear which may possibly occur is compensated by the follow-up of the piston part, hydraulic medium continuing to flow via the orifice receiving the nonreturn valve and located in the partition. An inadmissible excess pressure in the pressure space due to a temperature change or to tolerance-related variations in position between the piston part and the cylinder part is avoided by means of the overflow of hydraulic medium through the throttle bore.
In rapid, jolt-like steering movements and progressive wear of the matching steering mechanism parts (the rack is, for example, worn to a differing extent in its toothing region due to the different frequency of the meshing of the pinion into its teeth), the most unfavorable possible preconditions may coincide and cause the steering mechanism to jam, since the incompressibility of the hydraulic medium is such that a deflection of a thrust piece no longer takes place and the thrust piece acts in the same way as a rigid stop. An enlarged throttle bore, which would afford a remedy in this case, would be a disadvantage in normal operation, since damping would no longer take place.